Four different mechanisms for the failure of a tumor to stimulate a cytotoxic (CTL) response in allogeneic lymphocytes have been demonstrated. (a) Failure to activate a serum albumin factor required for CTL; (b) Failure to activate helper T-cells; (c) Absence of major histocompatibility antigen; and (d) Infection with a cytotoxic microorganism. We have shown that CaD2 is a non-stimulating tumor cell which will grow in allogeneic mice (C57BL/6) if freed of passenger macrophages but is immunogenic in syngeneic mice (DBA/2) if injected with proper adjuvants. We plan to produce a cell hybrid of CaD2 and an allogeneic stimulating tumor line, EL-4, and determine the tumorogenicity and immunogenicity of such a hybrid. We also plan to test the hypothesis that the growth suppressing property of tumors is due to a depletion of essential growth factors.